The present invention relates to embodiments of sourceless orientation sensors. The present invention teaches the measurement of all three orientation angles azimuth, elevation and roll in a fixed reference three-dimensional coordinate frame. The system utilizes a three-axis magnetometer to sense the Earth's magnetic field and thereby measure the azimuth and roll angles after the elevation angle has been determined by other means. The system uses the Earth's gravity vector, a rate sensor or a differential accelerometer pair or combination thereof to measure the elevation angle.
Much research has been made concerning sourceless orientation sensors. Most of these efforts utilize the Earth's magnetic field and a three-axis magnetometer to measure the azimuth angle after the roll and elevation angles have been determined by other means.
A first class of sourceless trackers utilizes fluid filled tilt sensors of various degrees of sophistication. The problem with use of liquid filled tilt sensors concerns their "slosh" effect, which becomes apparent during fast movements and quick stops. It can take more than one millisecond before the liquid stops "sloshing", resulting in considerable errors in angle determination. Newer systems use more effectively damped liquid filled tilt sensors where the "sloshing" effect is diminished, but still apparent.
A second class of sourceless trackers utilizes a two or three axis accelerometer and the Earth's gravity in order to measure the elevation and roll angles. These systems exhibit large errors during translation, since the accelerometers are also affected by translation accelerations.
A third class of sourceless trackers utilizes a two or three axis accelerometer and the Earth's gravity in order to measure the elevation and roll angles, when there is no translation. During accelerations, three rate sensors are used to measure the azimuth, elevation and roll angles. This means that three different technologies are employed. The rate sensors experience considerable drift and the accelerometers and magnetometer measurements compensate for the drift when the sensor is held still.
The following prior art is known to Applicants:
U.S. Pat. No. 5,526,022 to Donahue et al. discloses an example of the first class of sourceless orientation sensor as described above. The device utilizes an optical tilt sensor filled with a liquid and employs a float to measure the elevation and roll angles and includes a three-axis magnetometer which is used to measure the azimuth angle. The Donahue et al. system is affected by the "slosh" effect. The system does not utilize accelerometers or rate sensors. The present invention avoids the "slosh" effect.
U.S. Pat. No. 5,373,857 to Travers et al. discloses a three-axis magnetometer which measures the azimuth angle only as referenced to the Earth's magnetic North, where elevation and roll are determined from a tilt sensor. The system will not work if the sensor's roll angle is other than zero. Only measurements of rotation around a vertical axis may be made and the system only measures the azimuth angle. In contrast, the present invention measures azimuth, elevation and roll angles. The Travers et al. patent only teaches the use of a dual axis magnetometer to measure the azimuth angle for a Helmet Mounted Display. This scheme will only work if the elevation and roll angles are either zero or known by other means. The Travers et al. patent only teaches that simple liquid filled sensors can be used to measure these two other angles, elevation and roll and teaches the periodic calibration of the headset to a known calibration position (column 3, lines 36-45). The present invention carries out one initial calibration at the start of the operation and does not require any calibration, but will continue to output all three orientation angles relative to a fixed reference three-dimensional coordinate frame. The magnetic sensors are used in a first embodiment to measure both the azimuth and the elevation (or roll) angles. A second embodiment teaches the use of the magnetic sensors to measure the roll angle. It is also used for drift compensations. A third embodiment teaches the use of the magnetic sensors to measure the roll and azimuth angles. It is also used for drift compensations. The present invention does not have any "slosh" effect. It uses the magnetic sensor to measure orientation angles not restricted to one angle around a vertical axis and where roll and elevation angles can be of any value.
The "Interglide, Inertial Tracking System" developed by Eric Foxlin for Intersense and described in U.S. Pat. No. 5,645,077, utilizes three orthogonal solid state angular rate sensors, a two axis fluid inclinometer and a fluxgate compass (a three-axis magnetometer of the above-described first class). The angular rates in the sensor frame are integrated and translated to obtain angular orientation angles (azimuth, elevation and roll in a fixed reference three-dimensional coordinate frame) with instantaneous response, but slow drift. To compensate for drift, the tracker contains a compass and an inclinometer which periodically takes reference readings. Each time the user's head is motionless for a few moments, the tracker slowly resets itself to the orientation shown by the compass and the inclinometer. Variations of this system utilize a three-axis accelerometer of the above-described third class instead of the two-axis fluid inclinometer. The Foxlin patent does not teach the use of only two rate sensors to measure two orientation angles in the fixed reference three-dimensional coordinate frame. Equations 1, 2 and 3 clearly show that in order to translate the measurements taken with the rate sensors (in the sensor frame), one must know all three angle velocities in the sensor frame or assume that they are zero. FIGS. 8 and 10 of Foxlin show three rate sensors. FIG. 4C, item 462 shows the use of rate sensors and item 466 shows the conversion using the Equations 1, 2 and 3. If only two rate sensors are used, one must assume that the third rate angular velocity must be zero. If the roll velocity is assumed to be zero, then it is possible to translate all of the velocities to the fixed reference coordinate system and thereby find the three orientation angles. No other method is taught in the Foxlin patent. It is then possible to find all of the three orientation angles in the fixed reference coordinate system. It is not possible to measure only two angle velocities in the sensor frame and then only find two orientation angles in the fixed reference three-dimensional coordinate frame. (It is, however, possible to use only two rate sensors and find two orientation angles, but they are in the sensor frame.) All of the three orientation angles must be known at the time t in order to use Equations 1, 2 and 3.
In contrast to the teachings of Foxlin, the present invention utilizes only one rate sensor, a two-axis accelerometer and a two-axis magnetic sensor. The present invention measures the azimuth, elevation and roll angles in a fixed reference system. The present invention utilizes measurements of the Earth's magnetic field by magnetic sensors that can be used to determine two angles if one angle is determined by another means. The measurements of the Earth's field are not affected by translation acceleration.
The TRK300 system utilizes a three-axis accelerometer to measure the roll and elevation angles and a three-axis magnetometer to measure the azimuth angle. The system is of the second class as described above and is affected by translation acceleration.
The problems with prior art orientation sensors clearly demonstrate the need for a system that will overcome the inherent limitations with the "slosh" effect, large lag, complex and bulky systems. The need for such a system was the impetus for the development of the present invention, encompassing a simple, fast response orientation sensor.